There isn't really a best or a worst supplier, it's just a matter of where you live. Different networks serve different areas to differing degrees, you see.

Personally, I've been with O2, T-Mobile and 3 over the past two months while flipping between contracts, and my finding was that O2 were pretty good for reception but slow for mobile internet, T-Mobile were lousy for reception and mobile internet rarely even connected (and that's despite living and working in 'excellent' signal areas), while 3's reception has been excellent everywhere I've been, and internet speed's taken a serious step up. People will tell you the exact opposite for the areas where they live.

I think the best thing to do is to just experiment. Try all the networks, you get 7 days with each to cancel for bad signal problems. See what you can find.